1. Field of the Invention
The present invention relates to an image forming apparatus which is suitably applied to a color printer and a color copying machine both of a tandem type each having a photoconductor drum and further a color-shift-correcting mode and a color multifunctional machine composed of the aforesaid printer or the copying machine, and to an image forming method.
2. Description of Related Art
A color printer and a color copying machine both of a tandem type and a color multifunctional machine composed of the aforesaid printer or the copying machine have become popular in recent years. In each of these color image forming apparatus, there are provided a laser writing unit, a developing unit, a photoconductor drum, an intermediate transfer belt and a fixing unit, for each of yellow (Y), magenta (M), cyan (C) and black (BK) colors.
For example, the laser writing unit for Y color is arranged so that it draws an electrostatic latent image on the photoconductor drum based on information for color image forming. In the developing unit, toner for Y color is stuck to the electrostatic latent image drawn on the photoconductor drum to form a color toner image. The toner image is transferred onto the intermediate transfer belt from the photoconductor drum. The same process as in the foregoing is carried out for each of M, C and BK colors. The color toner image transferred onto the intermediate transfer belt is further transferred onto a sheet of paper, and then is fixed by the fixing unit.
In the image forming apparatus of this kind, it is essential, for maintaining the optimum quality of color image forming, to correct an image forming section so that color-shift may not be caused among yellow (Y), magenta (M), cyan (C) and black (BK) which reproduce R color, G color and B color of images in a document (hereinafter referred to as color-shift-correcting mode). The color-shift is generally caused by the tolerance for assembling of the writing unit and the photoconductor drum. It is considered that changes in temperatures in the open air or changes of temperatures in an apparatus resulted from the continuous use of the apparatus cause supporting portions for the laser writing unit and the photoconductor drum to be expanded or contracted to generate an aging positional shift thereof resulting in the color-shift.
With respect to the color-shift-correcting mode, a registration mark formed on the intermediate transfer belt or on a paper transfer belt is detected by a detecting means for detecting color-shift (hereinafter referred to a registration mark sensor) such as a reflection type sensor, and each amount of shifting for main scanning, sub-scanning, lateral magnification and skew for other colors is calculated for those of the reference color, whereby, image forming timing is adjusted, to correct the color-shift.
Concerning the color image forming apparatus of this kind, Patent Document 1 discloses an image forming apparatus, a method of controlling the apparatus and a memory medium. This image forming apparatus is provided with a pattern forming means, a pattern detecting means, a pulse width calculating means, a discriminating means and with a calculating means. In the course of calculating an amount of color-shift, the pattern forming means forms a color-shift detecting pattern on an endless-shaped belt through the photoconductor drum.
The pattern detecting means detects the color-shift detecting pattern formed on the endless-shaped belt, and outputs pattern detection signals to the pulse width calculating means. The pulse width calculating means calculates a pulse width by detecting rising time and falling of the pattern detection signals. The discriminating means compares a pulse width outputted from the pulse width calculating means with a pulse width threshold value established in advance, to discriminate the color-shift detecting pattern from a scratch on the belt. The calculating means calculates an amount of shift of each detected color for the reference color, based on the pattern detection data discriminated by the discriminating means from the color-shift detecting pattern.
If the image forming apparatus is constructed as in the foregoing, the color-shift detecting pattern can be discriminated precisely from a scratch on a belt, whereby, an amount of color-shift can be calculated by using only pattern detection data relating to the color-shift detecting pattern.
Further, Patent Document 2 discloses a color image recording method and an apparatus for the same. In this color image recording apparatus, electrostatic latent images for Y, M and C colors are formed on the photoconductor drum in the course of adjusting color balance, then, an electrostatic latent image for each color is developed by a developing unit for each color, and toner images each being of each color of Y, M and C colors are superposed each other on the photoconductor drum, thus, the superposed color toner image is detected by a toner concentration sensor.
A color balancing means inputs a toner concentration detection value obtained by the toner concentration sensor and an image density reference value established in advance to compare them to discriminate between abnormality and normality of image density. For toner of a color discriminated to be abnormal by the color balancing means, electrostatic latent images for the color are formed forcibly on the photoconductor drum, and the electrostatic latent images are developed by the developer for the color so that the toner for the color may be consumed.
If the color image forming apparatus is constructed as in the foregoing, a color image having excellent color balance can be reproduced on an appropriated image density, even in the case where image density is lowered for developing agents of each color, in spite of the toner concentration that is substantially the prescribed value.
(Patent Document 1) TOKKAI 2002-023444 (page 4, FIG. 1)
(Patent Document 2) TOKKAIHEI 05-323780 (page 5, FIG. 1)
A color image forming apparatus in the conventional example has the following problems.
i. When a white spot phenomenon is generated for some reasons on a registration mark, when practicing a color-shift-correcting mode in a color image forming apparatus shown in Patent Document 1 or in a color image recording apparatus shown in Patent Document 2, there is a risk that an amount of color-shift cannot be calculated precisely. The white spot phenomenon in this case is as follows; toner in a trace amount remaining on, and paper dust fine particles sticking unwontedly to, a photoconductor drum in the preceding cycle of charging, exposure, developing and cleaning, are not removed, and these remaining toner and paper dust fine particles prevent normal exposure in the following cycle of charging, exposure, developing and cleaning, resulting in the phenomenon that a part of the registration mark is developed to be white.
If this white spot portion of the registration mark is detected by the registration mark sensor, the mark is detected to be thinner than a width of the reference line in the design of the mark, resulting in a problem that an error is caused in a calculated value of a distance between marks, and in a risk that a precise amount of color-shift cannot be calculated.
ii. If the white spot portion is intermingled in the registration mark, the pattern detection data related to the registration mark having thereon the white spot portion are judged accidentally to be a crack on a belt, in Patent Document 1, and its data are excluded. Thus, the number of data is reduced by an amount corresponding to the excluded data, resulting in lowered reliability such as an average value.
iii. In accordance with Patent Document 2, in the course of adjusting color balance, electrostatic latent images for Y, M and C colors are formed on a photoconductor drum, then, an electrostatic latent image for each color is developed by a developing unit for each color, and toner images each being of each color of Y, M and C colors are superposed each other on the photoconductor drum, and thereby, the image density of the superposed color toner image is detected. Therefore, even when the color balance adjusting technology of this kind is applied on the color-shift correcting mode as it is, it is impossible to calculate a distance between respective colors for Y, M, C and BK. Further, on the color-shift-correcting mode, there is even a risk that toner of each color for respective colors of Y, M, C and BK are used wastefully, because toner for respective colors for Y, M, C and BK are not superposed.
In view of the foregoing, the present invention was attained.